Hello,
The problem is that you are getting permission denied when running OdfConverter:
sh: /var/www/html/billing/classes/../lib/OdfConverter/64/OdfConverter: Permission denied
You need to set the correct rwx access to the file. Anyway, we recommend you to use the conversion plugin based on LibreOffice to get the best output and it doesn't use OdfConverter to do the conversions, (https://www.phpdocx.com/documentation/conversion-plugin/conversion-plugin-installation-and-set-up).
Regards.